Reservoir Characterization Based on Hypocenter Location Analysis and 3-D Seismic Velocities

Abstract

Fluid injection process for hydraulic fracturing was conducted for two months from May 19-July 20, 2005 and was observed using the Micro-earthquake (MEQ) method. MEQ is a powerful method to assess the effectiveness of hydraulic fracturing treatments and to image the subsurface by delineating fluid in the reservoir for geothermal reservoir characterization. A total of 357 events of MEQ waveform data were used for this research from at least 17 installed stations. Initial MEQ locations were determined using Geiger’s method with Adaptive Damping (GAD) and two-layer 1D velocity models as input. Data were processed using VELEST’s software for updating event travel time and a 1D velocity model. VELEST’s output was used for input of travel time tomography application using SIMULPS12’s software. Total data used included 1,173 P-wave and 1,068 S-wave travel times. The tomographic method was applied to generate an image of the 3-D velocity structure and relocate hypocenters simultaneously. Our result show several prominent features: [1] hypocenters formed some clustering scattered, indicated as high permeability area; [2] strong low Vp anomaly, low Vs anomaly, and low Vp/Vs ratio at an elevation of 500 m above mean sea level (asl) were interpreted as related to the steam zone in the reservoir; [3] an underlying zone below the lower steam reservoir zone with low Vp, very low Vs, and high Vp/Vs ratio may be related to pore fluid pressure zone or the brine fluid zone. Based on the results, we can conclude that the tomographic method can give good information of hypocenter relocation and 3-D seismic velocity models for geothermal reservoir characterization. A steam zone at an elevation of 500 m above mean sea level (asl) and a brine zone below can be predicted from travel time tomography implementation.